Comparison of the Moist Parcel‐in‐Cell (MPIC) model with large‐eddy simulation for an idealized cloud

Abstract

The ascent of a moist thermal is used to test a recently developed essentially Lagrangian model for simulating moist convection. In this Moist‐Parcel‐In‐Cell (MPIC) model, a number of parcels are used to represent the flow in each grid cell. This has the advantage that the parcels provide an efficient and explicit representation of subgrid‐scale flow. The model is compared against Eulerian large‐eddy simulations with a version of the Met Office NERC Cloud model (MONC) which solves the same equations in a more traditional Eulerian scheme. Both models perform the same idealized simulation of the effects of latent heat release and evaporation, rather than a specific atmospheric regime. Dynamical features evolve similarly throughout the development of the thermal using the two approaches. Subgrid‐scale properties of small‐scale eddies captured by the MPIC model can be explicitly reconstructed on a finer grid. MPIC simulations thus resolve smaller features when using the same grid spacing as MONC, which is useful for detailed studies of turbulence in clouds. The convergence of bulk properties is also used to compare the two models. Most of these properties converge rapidly, though the probability distribution function of liquid water converges only slowly with grid resolution in MPIC. This may imply that the current implementation of the parcel mixing mechanism underestimates small‐scale mixing. Finally, it is shown how Lagrangian parcels can be used to study the origin of cloud air in a consistent manner in MPIC.

The moist parcel‐in‐cell method for modelling moist convection

“…The results shown compare closely with those obtained using the Met Office/NERC cloud model (MONC) run at more than double resolution (section 4.10 below and (Böing et al. , forthcoming)).…”

“…We next examine the behaviour of the new MPIC model, in particular its dependence on numerical parameter settings, and illustrate how well the model compares with a state‐of‐the‐art conventional numerical model (the focus of (Böing et al. , forthcoming)). For this purpose, we study the evolution of a moist, initially buoyant thermal located near the ground level.…”

“…Here, with L x = L y = L z we use an isotropic grid with n x = n y = n z = n g , and choose n g = 128 as the default; other values are discussed below and in Böing et al. , (forthcoming). As discussed at the end of section 3, the time step Δ t is adapted every time step to be inversely proportional to the maximum vorticity magnitude, i.e.…”

“…A detailed comparison with an entirely different numerical model, the Met Office/NERC cloud model (MONC), is the focus of Böing et al. , (forthcoming). Here we simply illustrate how well MPIC compares with MONC.…”

“…Comparisons with a convection‐permitting research model, the Met‐Office/NERC cloud model (MONC) are the focus of a forthcoming paper (Böing et al. , forthcoming). There, and in one figure here, the MPIC model is shown to compare well using significantly lower grid resolutions.…”